I. Field of the Invention
The present invention relates to an intravascular catheter or cannula assembly which is dwelled in a vein of a patient to feed blood to an extracorporeal blood circuit or to inject an infusion solution into the vein.
II. Description of the Prior Art
An intravascular catheter generally comprises a catheter hub, and a flexible catheter tube which has an axial through hole and the proximal end of which is fixed to the hub. In order to insert the catheter into a vein, a cannula is first inserted through the through hole of the catheter tube from the proximal end side thereof such that the pointed end of the cannula may project slightly from the distal end of the catheter tube. Then, the projecting pointed end of the cannula is pierced into the vein, and the catheter tube is inserted into the vein by means of the cannula acting as a guide. After the catheter tube is inserted into the vein, the cannula is withdrawn from the catheter tube.
The length of the cannula which projects from the distal end of the catheter tube, when the cannula is inserted within the catheter tube, must be small and must remain constant. For this reason, various means have been proposed for regulating the length of the cannula portion which projects from the distal end of the catheter tube when the cannula is inserted within the catheter tube. One such means is disclosed, for example, in Japanese Utility Model Application Disclosure No. 55-119739 (its claim and drawings are laid open to public inspection on Aug. 25, 1980). The means of this application comprises a cannula positioning rod of a predetermined length, one end of which is fixed to the outer surface of a tubular hub mounted on a cannula and the other end of which extends toward the proximal end of a catheter, and a stopper which is mounted on a hub of the catheter to abut against the distal end of the cannula positioning rod. When the cannula is inserted into the catheter tube, the cannula positioning rod abuts against the stopper to receive it, thereby defining the projecting length of the cannula. With this cannula projecting length regulating means, the projecting length of the cannula may be easily regulated without requiring complex procedures such as rotation of the cannula.
However, when an assembly with the cannula projecting length regulating means as described above is assembled, and the pointed distal end of the cannula is pierced into the vein of a patient while the operator holds the catheter hub, the distal end of the cannula is sometimes withdrawn into the catheter tube and may not allow piercing due to the piercing resistance of the skin of the patient. This is because the cannula positioning rod is only in contact with the stopper in this state. If piercing is forcibly performed, not only the patient receives a strong pain but also the vein may be needlessly damaged or broken. In order to prevent this, it is necessary to pierce the cannula into the vein while preventing the withdrawal of the cannula by holding the catheter hub with the thumb and the forefinger and/or the middle finger and simultaneously holding the cannula hub with the medical finger and the small finger. However, when these measures are taken, the movement of the cannula may not be completely prevented, and the distal end of the cannula may still be withdrawn into the catheter tube upon an attempt to pierce the cannula into the vein is made. Particularly when the distal end of the cannula is withdrawn into the catheter tube during the piercing process, this might lead to a fatal problem. Furthermore, these measures are not suitable for an operator with small hands.